Welcome to forums, this is Haziel part of the Zwift Support Team, please note that ERG mode is only available in workouts, and the SIM mode is only available in free rides.
In case you have issues with the ERG mode please check this article ERG Mode Troubleshooting, which covers the main reasons why you might have issue with the ERG mode, also be aware that you can turn the ERG mode on or off using the action bar once you’ve started your workout. Check out this article for more info on using the action bar.
If you have other questions please feel free to contact us.